[fix] codecov 2.1.12 was removed from pypi and later reinstated as 2.1.13

[estarfoo]

Cf. https://about.codecov.io/blog/message-regarding-the-pypi-package/

Description

Bump dev dependency codecov version from 2.1.12 to 2.1.13. Apparently the package was deprecated by its owner and removed entirely from pypi, then readded as a final, unmaintained 2.1.13 version. The hard dependency on 2.1.12 fails in the pipeline, and I hope that 2.1.13 would do the trick as drop-in replacement and help evaluate other PRs (which fail due to this issue).

Checklist

• This PR has sufficient documentation.
• This PR has sufficient test coverage.
• This PR title satisfies semantic convention. // (No release required just for this.)

Additional Comments

No additional comments.

[estarfoo]

@etimberg, would you prefer the pipeline issues to be addressed in this PR, or separately?

• https://github.com/tophat/ormar-postgres-extensions/actions/runs/8633161247/job/23776388434?pr=33: the import is unused (the code which was using it is commented out). Remove?
• https://github.com/tophat/ormar-postgres-extensions/actions/runs/8633161247/job/23776387921?pr=33: I have no idea where and how TWINE_USERNAME and TWINE_PASSWORD are being set in the workflow file, so I can't really debug that part, sorry.

[etimberg]

The import can be removed. Not sure about the TWINE stuff; I'm not at Top Hat anymore so it might be something that someone internally will need to fix